Facile Synthesis of Band Gap-Tunable Kappa-Carrageenan-Mediated C,S-Doped TiO2 Nanoparticles for Enhanced Dye Degradation

Semiconducting nanoparticles (SNPs) have garnered significant attention for their role in photocatalysis technology, offering a cost-effective and highly efficient method for breaking down organic dyes. Of particular significance within SNP-based photocatalysis are tunable band gap TiO2 nanoparticles (NPs), which demonstrate remarkable enhancement in photocatalytic efficiency. In the present work, we introduce an approach for the synthesis of TiO2 NPs using kappa-carrageenan (κ-carrageenan), not just as a reducing and stabilizing agent but as a dopant for the resulting TiO2 NPs. During the synthesis of TiO2 NPs in the presence of sulfate-rich carrageenan, the process predominantly leaves residual sulfur and carbon. The presence of residual carbon, in conjunction with sulfur doping, as indicated by fast FTIR spectra, XPS, and EDX, leads to a significant reduction in the band gap of the resulting composite to 2.71 eV. The reduction of composite band gap yields remarkable degradation of methylene blue (99.97%) and methyl orange (97.84%). This work presents an eco-friendly and highly effective solution for the swift removal of environmentally harmful organic dyes.


■ INTRODUCTION
An alarming 80% of the global population faces exposure to severe water pollution, marking a critical environmental challenge in the 21st century. 1 The textile and pharmaceutical sectors, utilizing metals and synthetic organic/inorganic compounds, pose significant risks to life.The wastewater discharged by these textile industries is frequently heavily colored with synthetic dyes and chemicals, posing severe health risks to humans and ecological harm to plants, animals, and microorganisms. 2,3This highly colored textile wastewater, primarily containing heavy metals, azo dyes, and other toxic substances, can disrupt essential biochemical processes like photosynthesis in plants and even lead to problems like eutrophication in aquatic environments. 4For instance, azo dyes, such as methylene blue (MB) and methyl orange (MO), make up around 70% of the commercial dyes utilized in the global textile industry. 5Approximately 50% of the synthetic dyes utilized in textile sectors are purportedly nonadherent to fabric substrates, thereby facilitating their leaching into the surrounding environment.This exposure introduces a broad spectrum of chemical and microbiological pollutants into aquatic ecosystems. 6,7These azo dyes are also known for their high toxicity and carcinogenic properties. 5Therefore, addressing the environmental impact of these pollutants is crucial for sustainable industrial practices and water quality preservation. 8−11 For instance, ASTPs are commonly used in wastewater treatment plants due to their cost-effectiveness but are ineffective in removing hazardous organic dyes. 12onversely, physicochemical treatments like coagulation and adsorption might raise environmental concerns due to their use of chemicals, high energy intensity, generation of sludge, and special resource requirements, which can have adverse ecological effects. 13On the other hand, membrane processes, such as membrane bioreactors, achieve high-quality water treatment through effective physical separation of solids and microorganisms but are energy-inefficient and expensive to operate. 13,14AOPs, however, hold great promise for treating textile wastewater containing dyes due to their capability to effectively break down soluble organic pollutants. 15,16urthermore, heterogeneous photocatalytic degradation-based AOPS utilizes low-cost semiconductor-based photocatalysts, thus offering a more cost-effective and highly efficient method for breaking down organic dyes compared with other AOPS. 17emiconducting nanoparticle (SNP)-based photocatalysis exhibits substantial potential for heterogeneous photocatalytic degradation-based AOPs owing to its simplicity, costeffectiveness, nontoxic nature, impressive degradation efficiency, and outstanding stability. 18−23 Among these, titanium dioxide (TiO 2 ) has garnered significant attention in photocatalysis. 22In 1972, Fujishima and Honda first suggested employing TiO 2 in photocatalysis, illustrating its capability to split water and generate hydrogen under light exposure. 24ubsequently, extensive research has been conducted on TiO 2 photocatalysis, catalyzing advancements in water 25 and air purification 26 technologies.Utilizing photocatalytic oxidation, this approach has demonstrated practicality in sewage treatment and air purification by decomposing pollutants and transforming organic substances into minerals, thus preventing further pollution. 27TiO 2 photocatalysts, among numerous photocatalytic semiconductor options, have garnered extensive research and application for producing hydrogen via photocatalysis and photodegradation applications.This is due to their exceptional attributes such as abundant availability, chemical durability, strong catalytic capability, resistance to photocorrosion, and nontoxic nature. 28espite the remarkable catalytic efficacy of TiO 2 , indicated by a band gap energy ranging from 3.0 to 3.2 eV, its limited solar energy absorption poses a challenge for efficient photocatalysis. 29This limitation arises due to the critical role of the electronic band structure and the band gap energy in determining the effectiveness of a photocatalyst.For optimal performance, the band gap energy should ideally be below 3 eV to facilitate broadened light absorption into the visible range and maximize solar energy utilization. 30Consequently, the implementation of strategies becomes imperative to enhance TiO 2 's photocatalytic activity.Researchers commonly use surface modification methods to bolster TiO 2 's adsorption capacity and reduce its band gap.These methods typically involve doping specific species or incorporating metal/ nonmetal dopants into the TiO 2 structure.
Specifically, nonmetal doping with elements like nitrogen, carbon, sulfur, fluorine, or iodine has proven effective in reducing the band gap and improving photocatalytic activity. 31oping TiO 2 with nonmetal elements modifies its electronic band structure, reducing the band gap energy and enhancing responsiveness to solar energy. 32Sulfur and carbon-doped TiO 2 , mainly, have garnered attention for their ability to decrease the band gap, highlighting significant potential for photocatalytic applications. 33,34arrageenan, derived from red seaweeds (Rhodophyta), consists of water-soluble polysaccharides and is a thickening ingredient in the food industry. 35Its notable gel-forming properties, from a high water-absorption capacity, make it conducive to constructing biohydrogels. 36Additionally, carrageenan hydrogels have served as an eco-friendly agent for reducing and stabilizing NPs under ultrasonic irradiation, with studies highlighting their effectiveness in controlling the size and shape of NPs. 37Comprising galactose and anhydrogalactose units connected by glycosidic bonds, carrageenans also feature a matrix rich in carbon and sulfur. 38Hence, carrageenan has emerged as a promising material, serving as a reducing and stabilizing agent for NP synthesis and as a source for carbon and sulfur as nonmetal dopants in TiO 2 .This leads to a reduced band gap and the production of highly active photocatalysts.
Chaudhary et al. initially demonstrated the application of various sulfate-rich carrageenans as direct sources for sulfur and carbon doping in TiO 2 , resulting in a highly active photocatalyst.The study further elucidated the impact of these seaweed carrageenans, specifically kappa, iota, and lambda, on the photodegradation of industrially essential dyes. 39While carrageenan has previously been utilized for synthesizing TiO 2 NPs for dye photocatalytic degradation, its potential for tuning the band gap energy of TiO 2 remains unreported.In this investigation, κ-carrageenan exhibited multifunctional properties by acting as a reducing and stabilizing agent and serving as a carbon and sulfur source for the doping process of TiO 2 nanoparticles.This diverse role reduced the band gap of the resulting nanoparticles, thereby enhancing its photocatalytic activity.The synthesized C, S doped TiO 2 nanoparticles (κC-TiO 2 NPs) underwent evaluation for their efficacy in degrading methylene blue (MB) and methyl orange (MO) in an aqueous medium, indicating a promising avenue for mitigating environmental contaminants present in wastewater.

■ RESULTS AND DISCUSSION
This research aimed to employ κ-carrageenan not only as a reducing and stabilizing agent but also as a source of carbon and sulfur for doping TiO 2 NPs, resulting in a reduced band gap and enhanced photocatalytic activity.To explore the impact of varying κ-carrageenan concentrations on the band gap energy of κC-TiO 2 NPs, three samples were produced at 0.1%, 0.2%, and 0.3% (w/v), designated as 0.1 κC-TiO 2 NPs, 0.2 κC-TiO 2 NPs, and 0.3 κC-TiO 2 NPs, respectively.Subsequently, the κC-TiO 2 NPs were assessed for their photocatalytic activity against MB and MO dyes and compared to commercially available pure TiO 2 NPs.
Powder XRD Analysis.XRD analysis was conducted to investigate the crystalline characteristics of produced κC-TiO 2 NPs, as shown in Figure 1 220), (215), and (303), respectively.The identified peaks closely mirror the characteristic tetragonal configuration associated with anatase TiO 2 , as delineated in the JCPDS Card No. 96−900−8214.Figure 1 demonstrates that all peaks align with JCPDS Card No. 96−900−8214, confirming the absence of contaminants or mismatched peaks. 40he NPs' average crystallite sizes were determined using Debye−Scherrer's equation: where d represents the mean diameter of the crystalline domains, λ denotes the wavelength, and β indicates the angular peak at the diffraction angle θ.
No observable diffraction patterns indicate the presence of carbon, sulfur, or other phases, suggesting that carbon and sulfur have been fully incorporated into the TiO 2 structure. 41,42esults demonstrate a decrease in crystallite size with increasing κ-carrageenan concentrations.The mean crystallite sizes were calculated to be 9.2, 8.7, and 8.0 nm for TiO 2 NPs synthesized with 0.1%, 0.2%, and 0.3% κ-carrageenan concentrations, respectively.These findings affirm that κcarrageenan facilitates the creation of a highly crystalline anatase phase of TiO 2 NPs.High crystalline indices (CI) of 92%, 90%, and 93% were calculated for 0.1%, 0.2%, and 0.3% concentrations, respectively, underscoring the pronounced crystallinity of the synthesized κC-TiO 2 NPs.The distinct XRD peaks observed in the case of κC-TiO 2 NPs indicate their distinctly high crystalline nature, which contributes to the improvement in the photocatalytic performance of the material.This can be explained by the improved movement of electrons from the conduction band of TiO 2 to the molecules adsorbed on the surface. 43V−Visible Spectroscopic Analysis.The light absorption characteristics of the κC-TiO 2 NPs synthesized at varying concentrations of κ-carrageenan were assessed by UV−vis spectra analysis across a wavelength range from 200 to 800 nm.This assessment was conducted using a suspension of κC-TiO 2 NPs in distilled water.The UV−vis analysis indicated a peak in absorption at 325 nm, as illustrated in Figure 2a.This absorption peak closely corresponds to the UV−vis band observed at 320 nm in TiO 2 NPs, as reported by Devikala and Abisharani. 44he absorption coefficient of TiO 2 NPs was calculated using eq 2: where α is the absorption coefficient, hv is the photon energy, and E g is the optical band gap energy.The E g for both commercial TiO 2 NPs and the synthesized κC-TiO 2 NPs at different κ-carrageenan concentrations is calculated using the Tauc plot shown in Figure 2b.This involves plotting αhv 1/2 against hv.The E g values for commercial TiO 2 NPs and κC-TiO 2 NPs synthesized with carrageenan at varying concentrations (0.1%, 0.2%, and 0.3%) are determined as 3.10, 2.97, 2.71, and 3.00 eV, respectively.These findings closely align with previously reported results, particularly in studies involving Kondagogu gum-mediated TiO 2 NPs, where the E g ranged from 2.99 to 3.18 eV. 45Figure 2b also illustrates that κC-TiO 2 NPs synthesized with κ-carrageenan exhibit comparatively lower E g values than pure commercial TiO 2 NPs.This observed change is attributed to residual carbon and sulfur from κ-carrageenan, which are incorporated onto the surface of the κC-TiO 2 NPs.Previous studies have demonstrated that the introduction of carbon and sulfur to TiO 2 NPs can result in a decrease in its E g . 33,34The decrease in E g is acknowledged for its significant benefits in boosting the photocatalytic performance of TiO 2 nanoparticles. 46Furthermore, a decrease in E g from 3.00 to 2.97 eV is evident as the κ-carrageenan concentration rises from 0.1% to 0.2%.However, as the κ-carrageenan concentration is further increased to 0.3%, there is a subsequent increase in E g .This observation is attributed to the likely saturation point reached at 0.2% κ-carrageenan concentration.
FT-IR Analysis.The FT-IR spectra of κC-TiO 2 NPs and pure commercial TiO 2 NPs in the 400−4000 cm −1 region are displayed in Figure 3.The peaks at approximately 3402.96 and 1646.53 cm −1 are ascribed to surface-absorbed water on the samples.These prominent bands emanate from the O−H stretching and bending vibrations of the chemisorbed/ physisorbed H 2 O molecules on the photocatalyst surface across all samples.The intensity of these peaks is relatively greater in κC-TiO 2 NP samples than in pure commercial TiO 2 NPs.This variation in peak intensity can be explained by the charge imbalance that occurs due to the surplus of charge upon substituting Ti ions with carbon and sulfur ions in the TiO2 lattice.. 47,48 This imbalance, in turn, draws additional hydroxide ions on the surface of the photocatalyst, leading to the formation of highly reactive surface-adsorbed OH.The reduction and stabilization of κC-TiO 2 NPs were evidenced by prominent absorption bands observed at 1040.05 and 1346.53 cm −1 .Specifically, the existence of vibrations attributed to the Ti−O−S bond is indicated by a peak detected at 1040.05 cm −1 , confirming the integration of sulfur into the lattice structure of TiO 2 . 49Additionally, the absorption at 1040.05 cm −1 may also indicate the presence of glycosidic linkage, providing further confirmation of the integration of carbon on the surface of κC-TiO 2 NPs. 50The appearance of a peak around 1346.53 cm −1 signifies the asymmetric stretching frequency of the S�O bond, indicative of the highest oxidation state of sulfur. 51This finding aligns with existing literature suggesting that the catalyst with sulfate exhibits both Bronsted and Lewis acidic sites, which are crucial for its photocatalytic activity. 52Hence, the two distinctive bands at 1346.53 and 1040.05cm −1 collectively characterize the features of sulfur and carbon doping on the surface of κC-TiO 2 NPs. 53Additionally, the strong band observed in the 400−800 cm O 1s energy level narrow scan detected two major peaks at binding energy centered at 531.4 eV and 529.1, ascribed to surface OH and Ti−O species, respectively, as shown in Figure 4b. 54Also, the Ti 2p narrow scan detected two major peaks centered at 458.1 and 464.5 eV and assigned to be the binding energy of Ti 3+ 2p 3/2 and Ti 4+ 2p 1/2 , respectively, as shown in Figure 4c. 54Significant carbon C 1s peaks were detected compared to commercial TiO 2 NPs, as shown in Figure 4d.It can be observed that the carbon peaks of κC-TiO 2 NPs slightly shifted to higher binding energies detected at ∼287 eV. 39urthermore, Figure 4(e-g) shows the narrow deconvoluted sulfur peaks at S 2p energy levels.It can be observed that all κC-TiO 2 obtained three (3) major peaks centered at binding energy range 164.4,165.1, and 165.8 eV assigned to the formation of S 0 , S−C, and C−SO x , respectively. 39,55,56This research further suggests that EDX and XPS spectroscopic analysis can confirm sulfur and carbon doping using κcarrageenan.
BET-BJH Analysis.The specific surface area and pore structure of commercial TiO 2 NPs and the synthesized κC-TiO 2 NPs were analyzed using N 2 adsorption−desorption techniques.Previous studies have highlighted that higher specific surface areas enhance photocatalytic activity. 57In Figures 5a, b, and c, the N 2 adsorption−desorption isotherms and corresponding pore size distribution of the synthesized curves of the synthesized κC-TiO 2 NPs are depicted.All samples exhibit similar sorption isotherms and pore size distributions.The specific surface area, pore volume, and pore size of the various samples are summarized in Table 1.The 0.2 κC-TiO 2 NPs demonstrate the highest BET surface area and BJH pore volume, suggesting an abundance of active sites conducive to efficient photocatalysis.
Morphological Analysis.After confirming the reduction, stabilization, and doping facilitated by κ-carrageenan, κC-TiO 2 NPs underwent microscopic characterization employing SEM and TEM techniques.SEM images for κC-TiO 2 NPs at varying κ-carrageenan concentrations are presented in Figures 6a, b,  and c.These images depict the spherical structure and the spherical morphology of the synthesized κC-TiO 2 NPs.Specifically, Figure 6a displays the formation of 0.1 κC-TiO 2 NPs with a larger particle size, whereas Figure 6b depicts 0.2 κC-TiO 2 NPs with decreased particle size compared with 0.1 κC-TiO 2 NPs.These observations align with the XRD data in Figure 1.The smaller particles are also prevalent in 0.3 κC-TiO 2 NPs (Figure 4c) but with the distinct presence of larger particles, thus broadening the particle size distribution.This phenomenon can be attributed to the excess κ-carrageenan functional groups interacting with one another rather than with the Ti ions, thus causing particle aggregation and increasing particle sizes. 37Notably, 0.2% κC-TiO 2 NPs exhibit the most uniformly distributed particle size among the three synthesized samples.This property can be attributed to the sufficiently homogeneous distribution of OH, sulfate, and glycosidic groups along the κ-carrageenan chain. 37The mentioned structural feature of κ-carrageenan is conducive to the efficient donation of electrons required to reduce, stabilize, and dope TiO 2 during NP synthesis.This observation corresponds with the results reported in the investigation conducted by Wan et al. in 2021, highlighting the significance of κ-carrageenan concentration in producing silver nanoparticles that are both small and uniform. 37Furthermore, the uniformity in the size and morphology of κC-TiO 2 NPs plays a crucial role in photodegradation processes. 58Smaller and uniformly sized TiO 2 NP photocatalysts have advantages in various aspects of photocatalysis. 59,60They possess higher light absorption efficiency due to a larger surface area that enhances the initiation of photodegradation reactions, 61 and exhibit better stability in photocatalysis, showing reduced tendencies for aggregation and sedimentation, thereby prolonging the catalyst's performance. 62Hence, the selection of 0.2% κC-TiO 2 NPs for the ensuing photocatalytic experiments was based on its advantageous attributes of small particle size and uniformity.Moreover, Figure 7 provides TEM images and particle size distribution of the κ-carrageenan-derived TiO 2 (0.2% κC-TiO 2 ) NPs.The TEM analysis unveils nanocrystalline TiO 2 NPs with spherical shapes and grain sizes ranging from 7 to 16 nm.These grain sizes agree with the conclusions drawn from the XRD data.
To further understand the characteristics of the κC-TiO 2 NPs, an EDX analysis was conducted.The EDX pattern of the codoped TiO 2 shows the presence of the dopants C and S in the sample, as shown in Figure 8.The distinctive C−S codoped TiO 2 spectra reveal the presence of Ti, O, C, and S elements.EDX analysis results indicated prominently intense peaks corresponding to Ti, alongside oxygen peaks, likely arising from dissociation of the precursor compound (titanium tetrabutoxide) or degraded κ-carrageenan employed in synthesis.
Mechanism for κ-Carrageenan-Mediated Synthesis of TiO 2 NPs.A sequential process can be delineated in synthesizing κ-carrageenan-mediated κC-TiO 2 NPs, consisting of three fundamental phases: reduction, growth, and stabilization. 62The process of synthesizing TiO 2 nanoparticles through the mediation of κ-carrageenan can be understood by examining the phases illustrated in Figure 9. First, Ti 4+ (red circles) ions are reduced in the reduction phase, and the resultant Ti atoms initiate nucleation.In this crucial stage, the retrieval of Ti 4+ ions occurs by interacting with κ-carrageenan from their salt precursor, titanium butoxide.The vinyl sulfonic acid groups on κ-carrageenan facilitate the absorption of Ti 4+ ions from the solution, enabling the anchoring of Ti ions to the −SO 3 -functional groups within the hydrogel network.The hydroxyl groups (OH − ) in the metabolites contribute to the reduction process by donating electrons, transforming Ti metal ions to a zerovalent state from a +4 oxidation state.Subsequently, these reduced titanium atoms undergo nucleation.Afterward, during the growth stage, small Ti nanoparticles naturally merge into larger ones through Ostwald ripening.This is facilitated by the greater binding energy among Ti metal atoms compared to atom-solvent interactions, thereby  increasing the thermodynamic stability of Ti nanoparticles.The final phase, stabilization, occurs as the nanoparticles assume their most energetically favorable conformation, heavily influenced by the κ-carrageenan's ability to stabilize the metal oxide nanoparticles.κ-carrageenan effectively caps the Ti nanoparticles, preventing further aggregation.After subsequent drying and calcination processes, the resulting κC-TiO 2 NPs (green circles) are obtained.Photocatalytic Activity of the Synthesized TiO 2 NPs.The study thoroughly investigates the photocatalytic activity of κC-TiO 2 NPs synthesized using κ-carrageenan as a reducing and stabilizing agent.Specifically, this investigation concentrates on their effectiveness in degrading methylene blue (MB) and methyl orange (MO) when exposed to UV light irradiation.These two organic dyes were selected for their noticeable color changes (Figure 10) before and after degradation, aiding in the confirmation of the reactions.
The photocatalytic efficacies of the synthesized κC-TiO 2 NPs were assessed for the degradation of MB dye when subjected to UV light irradiation (λ max < 400 nm) in dark conditions.Samples were gathered at consistent time intervals and subsequently tested in UV−visible absorbance spectra spanning from 200 to 800 nm.The prominent absorption band at 664 nm corresponds to the maximum wavelength of MB dye, revealing a discernible decline in intensity over time (Figure 11) in the presence of κC-TiO 2 NPs as the photocatalyst.The time-dependent electronic absorption spectrum of MB under UV light exposure (Figures 13a and 13b) demonstrates the minimal reduction in the absence of the biosynthesized nanocatalyst (dye degradation 1.8%).However, in the presence of κC-TiO 2 , particularly those synthesized with 0.2% κ-carrageenan, a remarkable degradation of MB occurs, with over 99% achieved after just 20 min of UV light exposure.This noteworthy efficiency underscores the outstanding photocatalytic properties of κC-TiO 2 NPs.Notably, no new absorption bands emerged during degradation, confirming the absence of stable reaction intermediates.This observation highlights that the degradation process proceeds directly from MB to its final degraded products, ensuring the eco-friendly removal of hazardous organic dyes, a crucial advancement in wastewater treatment and pollution control.
The study also investigates the degradation of MO, known for its characteristic UV−visible absorption bands at 464 and 272 nm.These bands are linked to the azo bond (−N�N−) and the phenyl ring of MO, respectively. 64The reduction in these absorption bands during photocatalytic degradation reflects the cleavage of azo bonds and phenyl rings, leading to the mineralization and degradation of MO.Figures 14a and  14b depict the time-dependent changes in the absorption spectrum of MO during visible light exposure with the existence of κC-TiO 2 NPs.Remarkably, within just 80 min of photoirradiation, κC-TiO 2 NPs achieve a 97% degradation of MO.This underscores the potency of κC-TiO 2 NPs in degrading complex organic dyes like MO, emphasizing their practical utility in environmental remediation.Indeed, the photocatalytic activity of κC-TiO 2 is comparable to or slightly  better than other TiO 2 -based photocatalysts reported in the literature (Table 3).After completing five cycles, it was observed that the percentage of MB dye degradation reached 94.8%, closely approaching the 99% achieved in the initial cycle.This finding validates the catalyst's high photostability during repetitive reactions, as illustrated in the reusability study in Figure 15.The marginal reduction in degradation percentage could be ascribed to the catalyst loss during collection in each step.
Mechanism of the Reduction of Band Gap.Nonmetal doping significantly contributes to boosting the photocatalytic efficiency of TiO 2 by modifying its band structure.In the typical photocatalytic process of TiO 2 , illumination induces the excitation of valence electrons, leading to their migration from the valence band (VB) to the conduction band (CB) and the subsequent production of electron−hole pairs.During this process, a positive hole h + is generated inside the valence band.The positive holes and liberated electrons undergo surface reactions on the photocatalyst in conjunction with water molecules that are adsorbed.Consequently, superoxide ions (•O 2 − ) and •OH radicals are produced during the reaction, facilitating the degradation of MB and MO dyes. 65he overall reaction can be represented as follows: To increase the photoresponse spectrum of TiO 2 , nonmetal ion doping was employed, particularly carbon and sulfur sourced from κ-carrageenan, to replace oxygen or partial oxygen in TiO 2 with nonmetals. 66This process reconstructs the VB of TiO 2 , shifting it upward and effectively reducing the band gap width (Figure 16).Consequently, the narrowed band gap allows TiO 2 to absorb a broader spectrum of light, thereby increasing the utilization efficiency of sunlight.Prior research has also highlighted that the inclusion of graphitic carbon results in an expanded surface area, thereby amplifying the adsorption and light absorption capabilities of sulfur-doped TiO 2 nanocatalysts. 39Furthermore, the modified band structure resulting from nonmetallic doping facilitates the effective disjunction of photoinduced electron−hole pairs.This separation is crucial for preventing recombination, as it impedes the loss of photoinduced charges before they can participate in the degradation reactions.The extended lifetime of these charge carriers enhances the availability of electrons and holes for redox reactions, such as the formation of •OH  from water molecules. 67Notably, the synergistic effect of a reduced band gap, efficient charge separation, and increased light absorption, especially with dopants like sulfur and carbon, results in a substantial increase in the photocatalytic degradation rate of pollutants.The improved photodegradation of model contaminants like MO and MB exemplifies this enhanced performance.

■ CONCLUSION
In this study, κ-carrageenan proved to be a versatile agent in synthesizing TiO 2 NPs with enhanced photocatalytic activity.Synthesis of TiO 2 Using κ-Carrageenan.The solution of κ-carrageenan was created by dissolving 2.5 g of κ-carrageenan in 250 mL of ultrapure water, followed by thorough stirring for 30 min at 60 °C until complete dissolution.To examine the impact of κ-carrageenan concentration on the synthesis of TiO 2 nanoparticles, three varying concentrations of κcarrageenan (0.10%, 0.20%, and 0.30%) were chosen for investigation.Subsequently, a solution containing 0.4 M titanium butoxide was gradually introduced into the κcarrageenan solution.The pH was adjusted to pH 9 through the incremental addition of 1 M NaOH.Vigorous stirring was maintained at 50 °C for 4.5 h.The resulting TiO 2 NPs, produced with distinct κ-carrageenan concentrations, were left to age for 24 h at room temperature (25 °C).The final TiO 2 NPs are produced through calcination at the temperature of 500 °C using a furnace for 2 h.
Characterization of TiO 2 NPs.XRD diffraction patterns of the powder were captured using a Cu Kα radiation source   (40 kV and 30 mA) within a 3−60°2θ range, employing 0.02°2 θ/0.60s on a Shimadzu XRD Maxima 7000 instrument from Japan.The phase of TiO 2 was confirmed by comparing the prominent positions of reported peaks with those in the standard JCPDS database.UV spectroscopy was executed using a Genesys 10S UV−vis spectrophotometer from Thermo Fisher Scientific.Quartz cuvettes and ultrapure water as the reference solvent were employed for the analysis, and the absorption spectra were documented in the wavelength span of 200 to 800 nm.Fourier Transform Infrared (FTIR) spectroscopy, conducted using the Shimadzu FTIR-ATR IR Tracer-100 instrument from Tokyo, Japan, was utilized to analyze the samples and identify the specific groups within the spectral range of 4000−400 cm −1 .The surface morphology of TiO 2 NPs was scanned by scanning electron microscope (SEM) (JEOL, JSM IT200).TEM analysis was performed using a Jeol TEM system (JEM 2100 Plus LaB6 TEM with STEM).X-ray photoelectron spectroscopic (XPS) analysis was conducted using a JEOL JPS-9200 spectrometer (JEOL Ltd., Japan) equipped with a monochromatized Al Kα X-ray source operating at 100 W under ultrahigh vacuum (about 107 Pa).The narrow scan spectra of oxygen (O 1s), carbon (C 1s), titanium (Ti 2p), and sulfur (S 2p) were obtained and corrected using the binding energy of adventitious carbon (285.0EV).All XPS spectra were deconvoluted with XPSPEAK version 4.1 using a true Shirley background and a 20−80% Lorentzian−Gaussian peak model. 68,69The specific surface area and pore size were analyzed using a BELSORP MAX X analysis instrument.
Photocatalytic Degradation of Dyes.The degradation of methylene blue (MB) and methyl orange (MO) through photocatalysis involved the addition of 100 mg of TiO 2 nanoparticles to 50 mL solutions containing 10 −3 M concentrations of MB and MO, respectively, under stirring.After reaching adsorption−desorption equilibrium within 16 h, the mixture was exposed to irradiation from four (4) 10 W UV−B lamps (λ < 400 nm) positioned approximately 10 cm away.Continuous stirring persisted until the reaction was deemed complete, as evidenced by an alteration in the color of the solution.For the kinetic analysis of MB and MO degradation, samples were obtained at uniform intervals and examined via UV−vis spectroscopy.To assess the reusability of TiO 2 NPs, photocatalytic degradation of MB was repeated three times under identical conditions.After each reaction, the catalyst was separated through centrifugation, washed with deionized water, and reused.

Figure 10 .
Figure 10.Color change illustration of (a) methylene blue and (b) methyl orange under different κC-TiO 2 photocatalysts compared with pure commercial TiO 2 NPs.
Varying concentrations of κ-carrageenan led to reduced band gaps and improved crystallinity in κC-TiO 2 NPs, as confirmed by XRD analysis, showcasing highly crystalline anatase-phase particles without contaminants.UV−visible spectroscopic analysis indicated a lowered band gap energy in κC-TiO 2 NPs in contrast to pure TiO 2 NPs, attributed to the incorporation of residual carbon and sulfur from κcarrageenan.SEM and TEM morphological analysis revealed that 0.2 κC-TiO 2 NPs exhibited the most uniformly distributed particle size.These nanoparticles, particularly the 0.2 κC-TiO 2 NPs, demonstrated superior photocatalytic efficiency in

Figure 16 .
Figure 16.Mechanism of methylene blue and methyl orange degradation through photocatalysis employing a κC-TiO 2 photocatalyst with a reduced band gap.

Table 2 .
Percentage of Degradation for MB and MO at Different TiO 2 NP Photocatalysts

Table 3 .
Performance of the Titanium Dioxide Nanoparticles Synthesized with Kappa-Carrageenan (κC-TiO 2 NPs) as a Photocatalyst in Dye Degradation Compared to Other TiO 2 -Based Photocatalysts Center for Sustainable Polymers, Mindanao State University − Iligan Institute of Technology, Iligan City 9200, Philippines Dan Michael A. Asequia − Center for Sustainable Polymers, Mindanao State University − Iligan Institute of Technology, Iligan City 9200, Philippines Carlo Kurt F. Osorio − Center for Sustainable Polymers, Mindanao State University − Iligan Institute of Technology, Iligan City 9200, Philippines Christine Joy M. Omisol − Center for Sustainable Polymers, Mindanao State University − Iligan Institute of Technology, Iligan City 9200, Philippines; orcid.org/0000-0002-9405-0070Andrei E. Etom − Center for Sustainable Polymers, Mindanao State University − Iligan Institute of Technology, Iligan City 9200, Philippines Renzo Miguel R. Hisona − Center for Sustainable Polymers, Authors Daisy Jane D. Erjeno − Mindanao State University − Iligan Institute of Technology, Iligan City 9200, Philippines